This invention relates to secondary, rechargeable lithium and lithium ion batteries, and more particularly relates to the synthesis of Li.sub.x Mn.sub.2 O.sub.4 intercalation compounds adapted for use as battery electrodes which provide extended high capacity.
On the basis of their economy, electrochemical efficacy, and environmental acceptability, Li.sub.x Mn.sub.2 O.sub.4 intercalation compounds have shown exceptional promise as electrode materials in secondary batteries for all manner of portable electrical power needs. Such materials have been used with outstanding success in positive electrodes for batteries comprising lithium metal, as well as in positive lithium source intercalation electrodes for lithium ion batteries comprising, for example, intercalatable carbon electrodes.
Li.sub.x Mn.sub.2 O.sub.4 electrode compounds have for some time generally been synthesized in a simple endothermic reaction between stoichiometric quantities of a lithium salt and a manganese oxide. Common precursors are, for example, the Li.sub.2 CO.sub.3 and MnO.sub.2 compounds discussed by Hunter in U.S. Pat. No. 4,246,253. The spinel in which the lithium content, x, nominally approximates 1 is shown by Hunter to be readily obtained by heating a 2:1 mole ratio mixture of Mn:Li at 800.degree.-900.degree. C. for a time to ensure thorough reaction, and then cooling to ambient working temperature, usually room temperature. Although Hunter's intent was the further acidifying reaction of the resulting LiMn.sub.2 O.sub.4 to derive a new .lambda.-MnO.sub.2 compound, the simple thermal reaction has been widely utilized to prepare the spinel, LiMn.sub.2 O.sub.4, for use in various types of battery electrodes.
In U.S. Pat. No. 4,828,834, Nagaura et al. noted the limited charge capacity exhibited by battery cells comprising electrodes fashioned of LiMn.sub.2 O.sub.4 prepared by Hunter's high-temperature method. The invention disclosed by Nagaura et al. was intended to improve the charge capacity of such batteries at the 3.2V intercalation plateau and entailed a variation in the Hunter process to essentially limit the reaction, or sintering, temperature of the Li- and Mn-source compound mixture to about 500.degree. C. The resulting unique material was, however, only partially crystallized at these lower temperatures and could not provide the higher potential capacity of the fully-crystallized spinel phase electrode. This is apparent from Nagaura's broad 2.degree. x-ray diffraction peak as compared with the approximately 0.1.degree. peaks observed in true crystalline spinel materials such as prepared by Hunter, or according to the present invention.
Thus there remained the dilemma of restricted choice between significant, but the short-lived capacity of Li.sub.x Mn.sub.2 O.sub.4 intercalation compounds synthesized at high temperature, or the more stable, but lesser initial capacity of such compounds processed at temperatures below their effective crystallization range.